Scoliosis is a medical condition in which a person's spine is abnormally curved and/or rotated. It is typically classified as congenital (caused by anomalies at birth), neurologic (occurring secondary to central nervous system disorders), or idiopathic (developing over time without definite cause). Idiopathic scoliosis is further sub-classified according to the age at which it occurs, earlier onset being associated with worse prognosis. Treatment of children with progressive scoliosis occurring at a young age is a difficult problem. Left untreated, progressive curves can produce significant deformity leading to deleterious effects on the developing heart and lungs resulting in a shortened lifespan.
Standard treatment for scoliosis includes spinal fusion surgery. This has limited use in younger children because of the potential alteration or cessation of spinal growth, which in turn can have adverse effects on axial growth, chest wall development, and lung development.
There are known methods to treat spinal deformities in the developing child that avoid spinal fusion. These include external bracing and surgery without spinal fusion. However, most early onset scoliosis is rapidly progressive and largely resistant to bracing, and compliance with brace-wearing regimens is generally very poor, which often makes surgical correction the preferred option.
Known non-fusion, growth-preserving surgical procedures include the placement of special spinal instrumentation known as growing rods. Growing rods are devices placed surgically within a patient's back that provide internal bracing in an effort to limit curve progression. An example of a prior art growing rod system 10 is shown in FIGS. 1-3. The system 10 includes dual, parallel growing rods 12 that are secured to vertebrae of a spine 14 above and below the deformity, thus spanning the curve being addressed. The growing rods 12 are secured to the spine 14 at foundation sites 16 by mounting hardware 18 (e.g., including mounting clamps, screws, and/or hooks) to form fixation constructs. Typically, two rods 12 are implanted in a parallel arrangement with one on each lateral side of the spine 14. Each rod 12 is typically composed of two independent rod segments that are longitudinally coupled together using tandem connectors 20. This configuration allows the rods 12 to be longitudinally adjusted (e.g., telescopically) at regular intervals to provide an overall increase in length.
Growing rod placement is an accepted technique that allows correction of deformity without preventing normal axial growth of the spine. This method requires frequent periodic lengthening of the rod system to adjust for longitudinal growth of the spine as the patient matures. Lengthening is performed by loosening the connectors, using a distraction device to push the rod segments apart until the appropriate amount of lengthening has been achieved, and retightening the connectors.
A fundamental strength of this existing growing-rod design over earlier treatments is also a significant weakness. Beneficially, serial lengthening allows the spine to grow. However, it also requires frequent returns to the operating room. Patients treated with this technique typically need repeat surgeries as frequently as every four to six months. This places the child at significantly increased risk for bleeding, infection, wound, and pulmonary complications. Additionally, overnight observation in a hospital is often necessary. Furthermore, young children with severe spinal deformities often have multiple other medical issues resulting in an overall compromised health status, and stress from repeated surgery can be overly burdensome on these patients and their families.
A second issue with current growing rod techniques relates to the timing of the expansion. Growing rods are generally left in place for a period of months before the patient is taken back to the operating room for lengthening. These interval periods allow the tissues surrounding the rods to heal, but also to scar. Scarred tissue within the telescoping connector parts is difficult and time-consuming to dissect, is more prone to infection, and complicates rod expansion. Scar tissue may also serve to further tether the growing spine, thus adding an additional deforming force. In addition, despite periodic lengthening, the interval placement of instrumentation can frequently result in cessation of spinal growth, which ultimately leads to premature fusion of the immature spine.
A third issue with current growing rod techniques relates to the expansion being only linear. When viewed from the side (the sagittal plane), the normal spine is a compound curve consisting of a lumbar curvature that is defined as lordotic or concave with respect to the ventral (front) surface of the body, a thoracic curve that is defined as kyphotic or convex with respect to the ventral surface of the body, and a cervical curve that is lordotic. The degree of curvature defines one's posture and the “sagittal balance,” which is the position of the head over the pelvis when viewed from the side.
All of the known growing-rod devices attempt to control curvature of the spine in a growing child using linear expansion. That is, as the spine elongates, the rods can be extended only linearly. Although the rods themselves can be bent and contoured somewhat (see FIG. 3), the expansion coupling is linear, so when the growing rod system is expanded the rods are moved only linearly. While this does provide some control of curvature in the coronal plane (front-to-back), this does not account for the natural curvature of the spine in the sagittal plane. Accordingly, when using known linear growing-rod systems, either spinal growth and alignment must be altered from their preferred normal curved state, or the fixation constructs (or another component of the growing-rod system) will fail.
If the rods and fixation constructs are strong enough to avoid failure, the spine will be forced to grow in a linear direction. This results in what is known as hypokyphosis or “acquired flatback deformity” of the thoracic spine. This affects the patient's overall sagittal balance and can result in what is known as negative sagittal balance in which the patient's head is centered posterior to its normal position thus negatively affecting overall posture, which results in chronic thoracic and lumbar pain. Even more potentially problematic for the patient is the possibility of junctional kyphosis. This occurs when the spine abnormally “kinks” at the end of the fixation constructs. With severe sagittal imbalance and hypokyphosis, it is thought that junctional kyphosis is much more likely to occur. This can result in catastrophic neurologic injury to the patient when severe and typically results in revision surgery.
If the fixation constructs are not strong enough or if the bone quality is poor, there is potential for construct failure or pullout from the bony foundation sites due to excessive stress on the system. This typically occurs at the more superior foundation sites on the thoracic spine. In the best case scenario, construct failure results in loss of spinal correction and revision surgery is thus required. In the worst case, the metal screws or hooks can pull out of bone and cause direct injury to the spinal cord resulting in paralysis or theoretically even death, the former having been reported in the medical literature.
Accordingly, it can be seen that a need exists for improved surgical instrumentation and methods for bone-deformity correction. It is to the provision of solutions meeting this need that the present invention is primarily directed.